The present invention relates to a monitoring system for motor vehicles that produces a warning signal when the pressure within a tire changes to an unacceptable level.
As is well known, the useful life of a tire is materially affected if the tire is operated over a prolonged period of time in an improperly inflated condition. Driving on an underinflated tire causes excessive flexing of the sidewalls of the tire that results in an inordinate buildup of heat in the tire and even may result in the tire becoming so hot that it will burst into flame. If the pressure in the tire falls to a sufficiently low level that the vehicle steers sluggishly, the operator probably will become aware that something is wrong with the vehicle and will stop and inspect the tires. However, if the degree of underinflation is only moderate, the operator of the vehicle may be become conscious of the underinflated condition of the tire until after the tire has been materially damaged.
When the vehicle is driven for prolonged periods of time in hotter climates, the hotter operating temperatures may cause the pressure within the tire to be increased to an objectionable high level at which excessive tread wear will occur and even may build up to such a high pressure that rupturing of the tire may result.
Various devices previously have been proposed for monitoring the pressure of a tire while in service and for producing a warning signal when the pressure of the tire changes from an acceptable level. Such devices usually have a sensor unit mounted on the wheel and an alarm component associated with the vehicle body and employ a purely electrical system or a combination of an electrical system and a mechanical system. A number of such systems depend upon a physical connection between the rotating wheel on which the sensor component is mounted and the alarm portion of the system associated with the vehicle body. Such physical connections, for example sliding electrical contacts, usually have not been found to be entirely satisfactory, however, since they can be rendered ineffective as a result of exposure to inclement weather or rough road conditions or merely because of wear of the contact surfaces after a prolonged period of use. Therefore, it is preferable that a system for monitoring tire inflation not rely on a physical connection for coupling the sensor circuit of the system mounted on the vehicle wheel with the alarm circuitry associated with the vehicle body. A number of monitoring systems that do not rely on a physical coupling between the sensor circuit of the system and the alarm circuitry have been suggested, some of which are described in U.S. Pat. Nos. 2,894,246; 3,093,812; 3,249,916; 3,374,460 and 3,602,884.
Many of the monitoring systems which previously have been proposed are objectionably complicated in their circuitry which not only materially adds to the initial cost of the system but also increases the number of potential sources for component failure. Use of circuitry that includes components within the sensor unit mounted on the wheel that require manual adjustment or "setting" to permit the monitoring system to function properly is objectionable since operation of the vehicle over rough roads may cause the component to be jolted out of proper adjustment and result in a malfunctioning of the system. Accordingly, it is desirable that the monitoring system be as simple in circuitry as possible with no variable components that require manual "setting" to permit the system to operate satisfactorily.